Cuatro Islas in Leyte: The pristine tropical paradise

 Protected Landscape/Seascape 

The four islands collectively known as the Cuatro Islas—Apid, Digyo, Himokilan, and Mahaba—were proclaimed on April 23, 2000, as a protected landscape/seascape under the NIPAS Act of 1992 by President Joseph Ejercito Estrada through Proclamation No. 270. The protected area is under the administrative jurisdiction of the municipalities of Inopacan and Hindang, Leyte.

Location of Cuatro Islas off the coast of Inopacan and Hindang, Leyte

 Geologic Origin 

The Cuatro Islas are believed to be remnants of a barrier reef system located off the coast of Inopacan and Hindang in Leyte. In a study conducted during the International Tropical Ecology Workshop in 1999 organized by the ViSCA-GTZ Tropical Ecology Program, we proposed that these coral reef islands are underlain by a volcanic basement connected to the extinct Mt. Sacripante on the Leyte mainland (Grenz et al., 1999). Geological evidence suggests that the islands formed during the Upper Pleistocene epoch of the Quaternary period, as indicated by the presence of coralline limestone of Quaternary age. During the Last Glacial Maximum, approximately 15,000 years ago, sea levels across Southeast Asia dropped by about 100 to 150 meters. This significant decline exposed large portions of the continental shelf, creating conditions favorable for coral growth in newly formed shallow tropical marine environments. In addition, ongoing tectonic uplift in Leyte and other parts of the Philippines further facilitated reef development, as corals thrive in shallow, sunlit warm waters. This combination of falling sea levels and gradual tectonic uplift contributed to the emergence and present configuration of the barrier reef system. Similar uplifted coral reef formations can also be found in the northwestern and southwestern regions of Leyte Island (Grenz et al., 1999). 

Digyo Island, a pristine tropical paradise

 Digyo, the Jewel of the Cuatro Islas 

 Among the four islands, Digyo stands out for its relatively flat, low-lying topography, rising only about 2 to 3 meters above the present sea level. It suggests that Digyo is geologically younger than the other islands, having formed more recently. Its crystal-clear turquoise water and fine white sand, derived from coralline limestone and the shells of marine organisms, make Digyo a favorite destination for tourists. Many people call it the jewel of the Cuatro Islas. 

The stunning white sandbar 

 Digyo Soils are Sandy and Undeveloped 

 The soils of Digyo Island are dominated by sand (largely calcium carbonate) and show no signs of pedogenesis. Roots of coconut trees, the main vegetation on the island, hold the sandy soil but contribute little humus. The soils belong to the Entisols order in Soil Taxonomy, and Arenosols in the World Reference Base (WRB). 

The young, sandy, & undeveloped soil of Digyo Island showing roots of coconut trees

Reference 

Grenz, J., Zukunft, S., & Asio, V. B. (1999). Geomorphology and soils of Apid Island, Inopacan, Leyte, Philippines. Annals of Tropical Research, 21, 1-8.

Soil Health Concept and Initiatives in the Philippines

What is Soil Health? 

Although not yet clearly defined, soil health has become a widely used term globally, even beyond the scientific community. This may be because the term “health,” defined by the Cambridge Dictionary as “the condition of the body and the degree to which it is free from illness, or the state of being well,” is easily understood by many people. By humanizing the condition of soil through the term “soil health,” issues such as soil degradation become more accessible and easier to understand for individuals from diverse backgrounds. 

A "healthy soil" used for intensive vegetable production in Cabintan, Ormoc, Leyte

Zethof et al. (2026) noted that the current popularity of soil health is unparalleled in the field of soil science. However, they question whether the term is merely a clever marketing strategy or if it has the potential to advance soil science beyond simple popularization. 

The Food and Agriculture Organization (FAO) has reported that the concept of a “healthy soil” has not yet been officially defined, despite being widely used for more than a decade. Soil health generally refers to the performance or functioning of soil, rather than its intrinsic physical, chemical, or biological properties. The Intergovernmental Technical Panel on Soils (ITPS) defines soil health as “the ability of the soil to sustain the productivity, diversity, and environmental services of terrestrial ecosystems.” 

A "sick soil" (unhealthy soil) due to salinity (seawater intrusion) in Matalom, Leyte

Soil health evolved from earlier, more technical terms such as soil quality and soil fertility. Soil quality is one of the three components of environmental quality, alongside water and air quality. While water and air quality are primarily defined by levels of pollution affecting human and animal health or natural ecosystems, soil quality is broader. It is defined as “the capacity of a soil to function to sustain biological productivity, maintain environmental quality, and promote plant and animal health” (Bünemann et al., 2018). In his book Pedologie oder allgemeine und besondere Bodenkunde, F.A. Fallou, one of the founders of soil science, introduced the term soil quality (Qualitas), although with a different meaning (Asio, 2005). 

A sick soil due to high acidity (soil pH below 4.5) in Quinapondan, Eastern Samar

Furthermore, soil fertility originated from the German term “Bodenfruchtbarkeit” and focuses primarily on crop production. According to the FAO, soil fertility is “the ability of the soil to supply essential plant nutrients and soil water in adequate amounts and proportions for plant growth and reproduction, in the absence of toxic substances that may inhibit plant growth.” 

Soil Health Initiatives in the Philippines 

In the Philippines, the National Soil Health Initiatives are being championed by Congressman Adolph Edward “Eddiebong” G. Plaza, the 2nd District Representative of Agusan del Sur. His soil health initiatives focus on several key areas, including the formulation of a national soil health strategy and the implementation of a “From Lab to Land” approach. This approach promotes the use of advanced soil testing laboratories and modern technologies, such as drones, to monitor soil moisture, fertility, and erosion risks. It also encourages crop diversification and land rehabilitation. 

Congressman Eddiebong Plaza addressing the participants of the Stakeholders' Forum
he organized on Dec 3-5, 2025

Congressman Plaza’s partners and collaborators include ACIAR-SLAM (Dr. Johnvie Goloran), Griffith University (Prof. Chengrong Chen), DOST-PCAARRD, the Department of Agriculture–Bureau of Soils and Water Management (DA-BSWM), Agusan del Sur State University (ADSSU; Pres. Joy Capistrano), Southern Leyte State University–Hinunangan (SLSU-Hinunangan; Dr. Ian Navarrete), and the Society for the Advancement of Philippine Soil Science (SAPSS; Dr. VB Asio). 

In support of this initiative, Congressman Plaza organized the Stakeholders’ Forum on Soil Testing Protocols and Information Systems, held on December 3–5, 2025, in Prosperidad, Agusan del Sur. 

References 

Asio, V. B. (2005). "Comments on" Historical development of soil and weathering profile concepts from Europe to the United States of America"." Soil Science Society of America Journal 69: 571-572.

 Bunemann, E. K., Bongiorno, G., Bai, Z. G., Creamer, R., De Deyn, G. B., de Goede, R. G. M., ... & Brussaard, L. (2018). Soil quality-A critical review. Soil Biology and Biochemistry, 120, 105-125. 

Zethof, J. H., Kalbitz, K., & Jungkunst, H. F. (2026). Soil Health—What Is It Good for?. Journal of Plant Nutrition and Soil Science.

The rock formation along the highway in San Jose, Dulag, Leyte

Have you ever wondered about the rock along the highway in San Jose, Dulag, Leyte? Especially where the road runs close to the sea, travelers see a striking, dark-colored rock formation exposed by the ongoing road widening. 

The andesite rock along the highway in San Jose, Dulag, Leyte

Formed by volcanism between 2.6 and 23 million years ago during the Miocene and Pliocene epochs, this rock in Dulag is known as andesite. Andesite is an intermediate type of volcanic rock, with silica (SiO2) content ranging between 52% and 63%, commonly found in areas with past or present volcanic activity. 

The Dulag andesite belongs to the Pliocene-Miocene intermediate rocks (Jahn & Asio, 2006)

Andesite got its name from the Andes Mountains in South America, where it is abundant. It is also widespread in volcanic regions around the world, especially along the Pacific Ring of Fire, where many volcanoes produce this type of rock. It is an extrusive (volcanic type) and the most widespread igneous rock in the Philippines. 

Andesite typically appears fine-grained, sometimes with small visible crystals embedded in it called phenocrysts. These crystals are often made of minerals like feldspar and dark-colored minerals such as pyroxene or biotite. Because of this, andesite can look gray, pinkish, or slightly dark, depending on its composition. 

Mineral composition of the andesite in Abuyog: Amp- amphibole; Cpx- clinopyroxene;
Opq-opaque minerals; Pl-plagioclase feldspar

Laboratory examination of thin sections using a petrographic microscope (above photo) revealed that the andesite in Dulag, Leyte, consists primarily of plagioclase feldspar (45% of the mineral content), clinopyroxene (20%), and amphibole (5%), along with minor amounts of secondary minerals. It is darker in color compared to the younger andesite rocks (Quaternary volcanics) in the central highlands of Leyte.

The typical andesite rock in the central highlands of Leyte formed by volcanism during the Quaternary period (2 million years ago up to the present). The sample comes
from Cabintan, Ormoc at an elevation of 900 m above sea level.

Acknowledgement

I thank the National Institute of Geological Sciences (NIGS) at UP Diliman for conducting the thin section analysis of my rock samples.

References

Britannica Editors. "andesite". Encyclopedia Britannica, 5 Jul. 2015, https://www.britannica.com/science/andesite. Accessed 3 April 2026.

Jahn, R., & Asio, V. B. (2006). Climate, geology and soils of the tropics with special reference to Southeast Asia and Leyte (Philippines). In Proceedings of the 11th International Seminar-Workshop on Tropical Ecology (pp. 21-25).

The Rock, Mineral, and Soil Collection at VSU: An essential Instructional Resource

The Rock, Mineral, and Soil Collection at the Pedology and Geoecology Laboratory of the Department of Soil Science, Visayas State University, Baybay City, is a popular educational attraction for students in Eastern Visayas. It contains hundreds of specimens, including different types of rocks (igneous, metamorphic, sedimentary), minerals (e.g. quartz, feldspar, calcite, amethyst, jade, jasper, mica, pyrite, etc), as well as sands and soils collected from different places in the Philippines and abroad. It is the only one of its kind in the Visayas and Mindanao.

In 2026, new additions to the collection include red sandstones from Utah (USA), peridotite and andesite rocks from Eastern Samar and Southern Leyte, and various mineral specimens donated by some alumni of the department.

Soil samples from across the Philippines are also on display. The collection is an essential resource for the teaching of soil science, agricultural science, environmental science, and earth science. Most soils (except peat soils) originate from weathered rocks, and the mineral composition of these rocks strongly affects soil properties and fertility. Rocks also serve as a source of nutrients that are gradually released into the soil through weathering.

A few of the many igneous rocks in the collection

Some of the metamorphic rocks on the display in the collection

Some samples of sedimentary rocks in the collection

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Note: The Rock, Mineral, and Soil Collection was established and is maintained by Dr. V.B. Asio. For more information or if you want to visit it, please email him at: vbasio@vsu.edu.ph

Landslides changed the soil characteristics in Leyte, Philippines

By Maria Cristina A. Loreño & V.B. Asio 

Landslide is defined as the downslope movement of soil mass, rocks, and debris. It is one of the most serious environmental hazards in the Philippines. On April 11, 2022, four catastrophic landslides occurred in Leyte due to tropical storm Agaton, which caused the loss of hundreds of human lives (for a detailed explanation of the causes, please see the Soil and Environment blog). Two of the landslides happened in Bunga and Mailhi in Baybay City. Until now, little research has been done on the effects of landslides on soil properties and soil development. Such information is crucial for the rehabilitation of landslide-affected areas. The objective of the study was to evaluate the changes in the morphological, physical, and chemical properties of volcanic soils due to landslides. 

The study was conducted in the Bunga landslide with old soil (Ultisol) and in the Mailhi landslide with young volcanic soil (Andisol). The sites are found on steep volcanic mountain slopes underlain by andesitic pyroclastic rocks. Vegetation in both sites is a mixture of trees, coconuts, and shrubs. Soil profiles were examined and sampled on the upper, middle, and lower portions of the landslides. The soil profiles on the upper slopes were not affected by the landslides and were used as reference (unaffected soil). Soil samples were collected from every soil horizon or layer and analyzed in the laboratory for physical and chemical properties.

 

Results revealed that the landslides changed many soil characteristics crucial to soil use and productivity. In particular, the kind and depth of soil horizons, soil color, abundance of plant roots, and presence of rock fragments were modified by the landslides. The trend was the same for both the old and young soils (Figs. 1&2). In Bunga with old soil, the landslide resulted in more clayey soil but with very irregular distribution with soil depth. In Mailhi, with young soil, the landslide led to the increased sand content in the soil profile (Fig. 3). 

Figure 1. Changes in soil morphology due to landslide in Mailhi, Baybay 

Figure 2. Changes in soil morphology due to landslide in Bunga, Baybay

Figure 3. Changes in the sand, silt, and clay contents with soil depth due to landslides.

As expected, landslides increased the soil's porosity due to the mixing and deposition of soil material. In terms of soil pH, the landslides increased the pH of both the old and young soils due to the mixing of the soil and the deposition of fertile topsoil from the upper slopes (Fig. 4). Landslides tended to decrease the soil organic matter (SOM) in the topsoil but increased it in the subsoils (Fig. 5).

Figure 4. Changes in soil porosity and pH due to landslide.

Figure 5. Changes in soil organic matter content with soil depth due to landslide.

Landslides changed the characteristics of the soils and the degree of soil development. The mixing of the soil made the soil unstable and prone to soil erosion and further slope failure. The landslides also lowered the fertility and potential productivity of the soils. Because of the instability of the soils, a few years should be allowed to pass before the landslide sites are utilized for agriculture, forestry, or other land uses.

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Note: This article is based on the poster presented by the authors at the 12th ASTHRDP Graduate Scholars Conference organized by the DOST-SEI and the National Science Consortium on 12-13 September 2024 at the Dusit Thani Resort Mactan, Lapu-Lapu City, Cebu. We thank the DOST-SEI for the ASTHRDP scholarship to MCAL and Dr. Luz Geneston Asio, and Mr. Kenneth Oraiz, GAC Members, for their valuable comments.

Some notes on the soils in the vegetable landscape of Benguet, Northern Luzon

Soils are formed from the weathering of rocks as influenced by climate, parent rock, topography, living organisms, and time. Among these factors, climate and topography appear to be the dominant factors that have influenced the properties and distribution of soils in Benguet, Northern Luzon. 

Benguet together with Abra, Apayao, Baguio City, Ifugao, Kalinga, and Mountain Province comprise the Cordillera Administrative Region (CAR). Benguet has a mountainous topography consisting of peaks, ridges, and canyons ranging in elevation from about 900m to 2,840m above sea level. 

The highest point of the Philippine highway in Cattubo, Atok, Beneguet

The subtropical highland climate (Cwb based on Köppen climate classification) with annual average highs of 25.3 °C in April and lows of 13.3 °C in January and an average precipitation of 1,829mm (Wikipedia) promotes moderate rock weathering and soil formation rates. The steep slopes on most mountain sides enhances rapid leaching and runoff, the latter results in severe soil erosion on cultivated and bare slopes. 

Steep slopes with young soils are terraced and planted to various vegetables

Most soils in Benguet have developed from diorite, an intermediate plutonic rock, as well as metavolcanics and metasedimentary rocks particularly slate. According to the published literature, the dominant natural vegetation of Benguet was the pine forest type. Compared with broadleaf forests, pine forests have lower soil organic carbon (SOC) contents, smaller labile carbon fractions, and lower amounts of SOC stocks. Moreover, pine forests tend to experience severe water erosion events (Nie et al., 2019. Catena 174: 104-111).

Outcrops of metasedimentary rocks in Atok, Benguet

The high soil erosion rates result in poorly developed and thin soils (Inceptisols). On more stable surfaces such as on summit positions, old soils can be found which may qualify as Ultisols. Regardless of the stage of soil development, most soils are acidic with pH below 5.0 (Laurean et al., 2015. Benguet State University Research Journal 74: 10-34).

Red and old soils on summit positions in the mountains.

Where intensive vegetable production is found, the landscape can be called Anthropocene landscapes due to the considerable soil and landscape modification resulting from human activities such as land use conversion from forest to agriculture, terracing, fertilizer and pesticide application, liming and others.

The beautiful Anthropocene vegetable landscape in Natubling, Buguias, Benguet.

In general, the rates of fertilizer and lime application by the vegetable farmers are not based on recommended rates. This necessitates soil fertility assessment of vegetable farms to be able to determine the appropriate rates of fertilizer and lime application for improved vegetable production. This is one of the objectives of our ACIAR SlAM Project (2020117) on managing heavy metals and soil contaminants in vegetable production led by Dr. Steve Harper of the University of Queensland, Australia.

Our ACIAR Slam Project Team from the Univ Queensland, UPLB, BSU, VSU & USTP

What caused the Baybay landslides during the Tropical Depression Agaton on April 11, 2022?

The Baybay landslides on April 11, 2022, brought about by the Tropical Depression Agaton, have already claimed 116 lives, and many victims are still missing. Social media is buzzing with explanations about what caused the landslides. As usual, deforestation is claimed as the top culprit. And as always, the deforestation issue is politicized. But is it the cause of the landslides? Several factors cause landslides. In the case of the Baybay landslides, the most important are geology (rock type), topography (slope), soil characteristics, land use (and vegetation cover), and rainfall. Let me explain the role of each factor.

Geology

All the landslides in Baybay are located on the western slopes of the central highlands of Leyte, also called the Leyte Cordillera. This mountain range is volcanic, and the rocks consist mainly of pyroclastic rocks, specifically basalt and andesite. Pyroclastic rocks are fragmented or unconsolidated rocks produced by volcanic eruptions. Consequently, the slopes underlain by these materials are generally weak and prone to slope failure. This situation is aggravated by the presence of the Philippine fault line along the central highlands, which has caused the shearing of the rocks.

The common type of pyroclastic rocks in the central highlands of Leyte

Topography

The western slope of the central highlands is generally rugged and mountainous. The steepness of a mountain slope is a major determining factor in whether the slope will fail or not. The steeper the slope, the less stable it is. In the presence of a triggering factor such as a heavy rainfall event, steep slopes (>25%) may fail, thereby causing landslides. From the topographic maps available on the internet, one can easily see that the source areas of the Baybay landslides have steep slopes. 

Topographic maps show the steep slopes of the source areas
of Bunga & Kantagnos landslides

Soil

Soils vary in the stage of development from young (poorly weathered) in the plains to old (highly weathered) soils in the mountains. Young, very porous, and unstable volcanic soils (65% porosity) prone to land sliding are widespread in the upper mountain slopes, generally above 300 meters elevation, such as in Mailhi. Except for the Mailhi landslide, most of the Baybay landslides occurred on the old and highly weathered soils. These soils are highly friable, clayey, and prone to shallow landslides. When saturated with water, the clay serves as a lubricant for the sliding mixture of soil and rock debris. And also, regardless of soil type, the soil can turn into a liquid state when supersaturated with water resulting in mudslides.

The source area of the Bunga landslide with its highly weathered soil, deep-seated
characteristic, and mixed vegetation cover. (Photo Source: jbatravelvlog)

The source area of the deep-seated Kantagnos landslide with its highly weathered soil and relatively thick mixed vegetation. (Photo Source: Dan Michael Castanares)

The deep Mailhi landslide with its unstable young volcanic soil and mixed
vegetation cover. (Photo Source: jbatravelvlog)

Land use 

Vegetation cover, particularly trees, can prevent soil erosion and shallow landslides, which generally occurs within the root zone. No doubt, forest trees can minimize shallow landslides better than shallow-rooted plants like coconut and grasses like cogon. Studies have revealed that roots increase water permeability and the mechanical stability of shallow pyroclastic soil cover (Alfonso-Dias, 2019). Zhang et al. (2019) found that the 23-year-old reforest in the mountain in Tacloban, Leyte, positively affected the hillslope hydrological functioning. But deep landslides that occur below the root zone are beyond the control of the roots of the vegetation cover (Zhuang et al., 2022). In such a case, the failure of the land surface is controlled more by the steep slope and by the weak geological and soil foundation. This explains why landslides can occur under any type of vegetation cover or land use. For example, on a clear day, one can see several old landslide scars in the forest on Mt. Pangasugan. In the Bunga and Kantagnos landslides, the largest Baybay landslides, the source area in the upper part of the mountain is still covered with mixed vegetation consisting of trees and coconuts. And according to Forbes and Broadhead (2013), the forest cover will not affect the occurrence of landslides during extreme events such as heavy rainfall.

Rainfall

Excessive soil water content from heavy rainfall is generally considered the primary cause of slope failure (Forbes and Broadhead, 2013). The tremendous volume of rain dumped by Agaton in three days from April 9 to 11 was 907mm, which is one-third of the annual rainfall in Baybay (Source: VSU-PAGASA Agromet Station). This is close to a meter deep water poured into the land surface in Baybay in only 72 hours. This volume of water is equivalent to 9,000 cubic meters of water per hectare or 200 gallons per square meter. No vegetation type can absorb and evaporate this tremendous amount of water in so short a time. Likewise, no soil can either hold or percolate this volume of water in just 72 hours. Even the very porous young volcanic soil in Mailhi, which has an average porosity of 65% and moisture content at a field capacity of 40%, can only potentially absorb 58 gallons of water per square meter calculated to a depth of 1 meter. The old soil under the forest can potentially hold a maximum of only 61 gallons per square meter to a depth of 1 meter, while the old soil under coconut can hold only 55 gallons per square meter. These values are based on the assumption of a flat surface and fast infiltration rates which are not the case in the landslide areas. This means that the actual amount of water that the soils on the mountain slopes can hold is much lower than the values mentioned. As soon as the soil is saturated with water, the incoming rainwater cannot be accommodated in the soil pores and flows as surface runoff, causing the landslides and the flooding, for the first time in about four decades, the city center of Baybay. 

Severe flooding of the city center of Baybay due to Agaton on April 10, 2022.
This is the first severe flooding of Baybay that I have witnessed in
nearly four decades. (Photo Source: Discover Baybay City).

Summary

The Baybay landslides were triggered by the extremely heavy rainfall and enhanced by the unstable and highly weathered soils (or the young unstable volcanic soil in the case of Mailhi), weak geologic material composed of unconsolidated pyroclastic rocks, and the steep slopes. The role of vegetation cover is not straightforward since it can partly prevent shallow landslides but not deep-seated landslides such as those in Bunga, Kantagnos, and Mailhi. This means that the coconut's shallow root system (about a meter deep) may not have effectively prevented the landslides, but the same can be said of the trees with their 2-3 m deep rooting zone since the landslides are several meters deep in their source areas. Landslides are natural geologic processes on the land surface. Catastrophic landslides like those we have witnessed in Baybay may occur again anytime at any place with the above environmental conditions regardless of the vegetation cover. I suspect that several incipient landslides (landslides in the initial stage) were produced in various areas during Agaton but are hidden by the vegetation cover. Another typhoon may trigger these incipient landslides to become full or even catastrophic landslides. Thus, it is crucial that residents living in landslide-prone areas be given proper and timely advice. 

References:

Alfonso-Dias 2019. Dissertation, University of Montpellier, France.

Forbes, K. and J. Broadhead. 2013. RAP Publication 2013/02. FAO Regional Office, Bangkok.

Zhang et al. 2019. Geoderma 333: 163-177.

Zhuang et al. 2022. Engineering Geology 298